1. Field of the Invention
The present invention relates to a fixing apparatus utilizing induction heating as a heat source, a heat fixing apparatus for heat fixing a toner image, formed on a recording sheet, with a thermally fusible material such as toner as a developer, onto such recording sheet, and an image recording apparatus of electrophotographic system such as a copying apparatus, a printer or a facsimile, utilizing such heat fixing apparatus.
2. Related Background Art
In an image forming apparatus such as an electrophotographic apparatus, there is provided image forming means which utilizes a developer (toner) to form a toner image on a recording sheet, and the recording sheet bearing the toner image is conveyed, by sheet conveying means, in a heat fixing apparatus 801 shown in FIG. 14, in a direction indicated by an arrow whereby the toner image 811 is fixed by heat and pressure onto the recording sheet 810.
In the heat fixing apparatus 801, a heating roller 802, opposed and pressed to a pressure roller 803, is provided therein with a halogen heater 804 as a heat source, and the pressure roller 803 and the heating roller 802 are rotated in a direction indicated by an arrow by an unrepresented drive source.
FIG. 15 illustrates a temperature regulation circuit for regulating the surface of the heating roller at a predetermined temperature, wherein a control signal generation unit 901 compares a detection output of a temperature sensor 905 with a reference signal Vr by a comparator 904 to output an on/off signal, and a switching unit 902 turns on and off a connection circuit for the halogen heater 804 and an AC power source 903 in response to the aforementioned on/off signal thereby regulating the temperature of the halogen heater 804.
In the above-mentioned temperature regulation system for the heating roller 802 by the on-off control of the halogen heater 804, since the system is based on indirect heating of the heating roller 802 by radiation heat, a part of the generated heat is lost by convection heat and the amount of heat radiation becomes large because the entire heating roller is uniformly heated, so that the system is associated with drawbacks of a very poor heat efficiency and a large electric power consumption.
Also in order to obtain stable fixing ability, the fluctuation in temperature of the heating roller 802 has to be made small, whereby a heating roller 802 of a large heat capacity is required as a result. Consequently, in case of heating the heating roller 802 to a fixable temperature with an electric power available within the electric power consumption of the entire apparatus in a situation where the temperature of the heating roller is considerably lower than a target temperature, there is encountered also a drawback of requiring a long time, in relation also to the poor heat efficiency of the indirect heating.
An induction heating system is proposed for solving such drawbacks and realizing a high efficiency and a high-speed temperature elevating characteristics. In such induction heating system, a high frequency magnetic field generated by supplying an excitation coil with a high frequency current is applied to a surfacial layer of the heating roller to generate an eddy current in an electroconductive layer at the surface of the heating roller, thereby directly heating the heating roller by a Joule""s heat generated by such eddy current, whereby a high heat generating efficiency is realized and a high-speed temperature elevation is rendered possible. Also, since the amount of heat generation is proportional to the supplied electric power, the temperature of the heating roller can be controlled promptly and easily by a variable control of the electric power supply, and the electric power can be efficiently utilized to attain a saving of the electric power.
However, in order to supply the coil with a high frequency current, there is a method of converting a commercial alternating current of a low frequency into a high frequency, but such conversion of a low frequency into a high frequency is undesirable because the configuration of the electrical circuit inevitably becomes complex. Also in a fixing apparatus in which a recording material is passed, the heating medium has to be maintained at a predetermined temperature while the heat movement takes place frequently, so that a highly precise current supply is required.
For these reasons, there is being desired a method capable of generating a high frequency current by a simple electrical circuit.
An object of the present invention is not only to generate a high frequency current with a simple configuration but also to achieve further saving of the electric power while maintaining an appropriate temperature.
Another object of the present invention is to maintain an appropriate surface temperature of a heating medium while enabling a zero-voltage switching even in an environment where the electric power supply is reduced.
Still another object of the present invention is to enable a zero-voltage switching regardless of a fluctuation in the input voltage, thereby suppressing an electric power loss in a semiconductor switching element.
Still another object of the present invention is to provide an apparatus and an image forming apparatus, including a coil for generating a magnetic field, a heating medium provided with an electroconductive layer which generates heat by an eddy current caused by the magnetic field generated by the coil and is adapted to fix an unfixed toner image on a recording material by heat, a temperature detecting member for detecting the temperature of the heating medium, a semiconductor switching element for on-off control of a current supply to the coil, and a capacitor electrically connected in parallel to the coil, wherein the capacitor executes charging and discharging and the turn-on time from the semiconductor switching element to the coil is controlled according to the output of the temperature detecting member.